This invention relates to a method and apparatus for improving AC transmission system dispatchability, system stability, and power flow controllability using DC transmission systems. Consolidated load centers (e.g., large urban areas or geographical areas with significant contiguous load) are typically served through a local AC network by a combination of local generation (i.e., inside or immediately adjacent to the load center) and long distance, high-voltage AC or DC transmission lines from distant generation sources. Often, the area is enclosed or partially enclosed by a high voltage AC ring. In some cases, power destined for other, distant load centers is fed, or “wheeled” through the local AC network. This is illustrated schematically in FIG. 1. This arrangement is vulnerable to voltage stability and service interruptions due to outage or fault events on one or more of the long distance lines serving the area, which subsequently cascade into the local area. Additionally, it is difficult to control the loading of either the long distance lines or the local network. Typically this would require the addition of expensive AC controllers such as the Unified Power Flow Controller (UPFC) or other flexible AC transmission systems (FACTS) devices. The regional grid or local load center in this situation is not isolated and not controllable.
A partial solution would be to serve the regional network or load center entirely by small, distributed generation units located near the loads, and connected to the network and controlled in a unified fashion, such as via an AC ring, as illustrated in FIG. 2. Generators would be dispatched as needed to serve nearby load and could be ramped up and down throughout the load cycle. There would be less vulnerability to events on distant transmission lines. However, such a solution has many difficulties, including the cost of small generators and the difficulty siting them in populated areas. Moreover, this solution does not provide for the wheeling of bulk power through the area.